Embodiments relate generally to transport refrigeration systems, and more particularly to multi-compartment transport refrigeration systems using one or more evaporator isolation valves.
The refrigerated container of a truck trailer uses a refrigeration unit for maintaining a desired temperature environment within the interior volume of the container. A wide variety of products, ranging for example, from freshly picked produce to deep frozen seafood, are commonly shipped in refrigerated truck trailers and other refrigerated freight containers. To facilitate shipment of a variety of products under different temperature conditions, some truck trailer containers are compartmentalized into two or more separate compartments each of which will typically have a door that opens directly to the exterior of the trailer. The container may be compartmentalized into a pair of side-by-side axially extending compartments, or into two or more back-to-back compartments, or a combination thereof.
Conventional transport refrigeration units used in connection with compartmentalized refrigerated containers of truck trailers include a refrigerant compressor, a condenser, a main evaporator and one or more remote evaporators connected via appropriate refrigerant lines in a closed refrigerant flow circuit. The refrigeration unit must have sufficient refrigeration capacity to maintain the product stored within the various compartments of the container at the particular desired compartment temperatures over a wide range of outdoor ambient temperatures and load conditions.
In addition to the afore-mentioned main evaporator, one or more remote evaporators, typically one for each additional compartment aft of the forward most compartment, are provided to refrigerate the air or other gases within each of the separate aft compartments. The remote evaporators may be mounted to the ceiling of the respective compartments or mounted to one of the partition walls of the compartment, as desired. The remote evaporators are generally disposed in the refrigerant circulation circuit in parallel with the main evaporator and share a common compressor suction plenum. When two or more compartments cool simultaneously in a system with a common suction/evaporation plenum the saturated evaporation temperature is shared between all compartments and coils. The resulting common evaporating temperature is dictated by coldest temperature compartment. Although simplistic, it creates a very inefficient refrigeration cycle.
When two different temperature compartments cool simultaneously on a common evaporation plenum the evaporator for the lowest temperature compartment (e.g., a frozen food compartment) can become a condenser instead of an evaporator and reject heat from the higher temperature compartment when the perishable or higher temperature compartment is trying to cool. A temperature rise of the frozen compartment when a perishable compartment is active is greater than if the frozen compartment was simply turned off. This is due to the fact that condensing latent and sensible heat exchange is happening within the frozen compartment evaporator as the perishable compartment evaporator is trying to cool. When the higher temperature compartment is ordered to cool, the frozen compartment sensed superheat becomes negative due to the pressure rise from higher temperature compartment flow. The frozen compartment expansion valve shuts and temperature rise in the frozen compartment evaporator is significant due to latent and sensible heat exchange as the vapor from the perishable compartment evaporator is re-condensing within the tubes of the frozen compartment evaporator. In order for the saturation pressure of the system to increase, the absolute coil temperature increases in the frozen compartment evaporator generating unwanted heat in the frozen compartment. A significant amount of frozen cooling time (e.g., running an engine and compressor) is spent recovering from the pulsed cooling resulting in net heating effect in the frozen compartment. Additionally this causes a very cold perishable evaporation temperature and significantly more ice formation on the perishable compartment evaporator.